1. Field of the Invention
The present invention relates to a UV synthetic silica glass optical member and a reduction projection exposure apparatus using the same and, more particularly, to a synthetic silica glass optical member suitably used for a lens of an imaging optical system, the substrate of a photomask such as a reticle used to print the circuit pattern of an integrated circuit, a diffraction optical element (DOE), and an etalon plate for a light source in a reduction projection exposure apparatus using UV light such as an excimer laser beam, and a reduction projection exposure apparatus using the synthetic silica glass optical member.
2. Background Art
Conventionally, to transfer the pattern of an integrated circuit such as an IC or LSI, a reduction projection exposure apparatus (or photolithography apparatus) is mainly used. Along with the recent increase in integration density of integrated circuits, a wider exposure region and a high resolution over the entire exposure region are required for the projection optical system of an apparatus. To increase the resolution of the projection optical system, improvements of the apparatus have been examined, including shortening the exposure wavelength and increasing the numerical aperture (NA) of the projection optical system.
In the technique of making the exposure wavelength shorter, the light source used for an apparatus changes from g-line (wavelength: 436 nm) to i-line (wavelength: 365 nm), KrF excinier laser (wavelength: 248 nm), or ArF excimer laser (wavelength: 193 nm). To further increase the integration density of integrated circuits, use of a light source such as an F2 laser (wavelength: 157 nm), X-rays, or electron beam has been examined. Of these light sources, a reduction projection exposure apparatus using the F2 laser can be manufactured on the basis of the conventional design concept and therefore has recently received a great deal of attention.
On the other hand, an optical member used for the optical system of the apparatus is required to have sufficient optical characteristics for use together with light having such a short wavelength. Generally, a reduction projection exposure apparatus using light having a longer wavelength than i-line uses multi-component optical glass as the lens member of its illumination optical system or projection optical system. However, such optical glass abruptly decreases its internal transmittance for light having a shorter wavelength than i-line and, especially, rarely exhibits transmittance for light having a wavelength of 250 nm or less. For this reason, as materials of a lens of the optical system in a reduction projection exposure apparatus using an excimer laser as a light source, synthetic silica glass and single-crystal calcium fluoride (CaF2) are used. These two materials are necessary for chromatic aberration correction in the imaging optical system of an excimer laser.
Of the above optical materials, synthetic silica glass has a high light transmittance and, additionally, excellent characteristics: satisfactory excimer laser resistance, resistance to a change in temperature, satisfactory corrosion resistance and elastic performance, and small linear expansion coefficient (about 5.5xc3x9710xe2x88x927/K) near room temperature. Hence, for a reduction projection exposure apparatus, synthetic silica glass has been examined as a material of an optical member for which optical characteristics are required, including satisfactory UV resistance, and low possibility of heat generation in the substrate and thermal expansion due to the heat. A detailed example of such an optical member is the substrate of a photomask such as a reticle that is one of important elements in printing a pattern on a wafer.
Conventionally, silica glass is synthesized by a soot method such as VAD, OVD, or sol-gel method, or a direct method. To synthesize silica glass, a silicon compound such as silicon tetrachloride, silicon tetrafluoride, or organic silicon compound is used as a material. When a component such as germanium (Ge), titanium (Ti), boron (B), fluorine (F), or aluminum (Al) is doped as needed, the physical properties of silica glass can be changed. In Japanese Patent Application Laid-open No. HEI 6-305736, a method of manufacturing synthetic silica glass as a material of a lens used together with an excimer laser beam is disclosed. However, generally, an optical system such as a projection optical system or illumination optical system is constructed by not a single lens but a plurality of lenses (lens group), so the light transmittance of the entire optical system corresponds to the integrated value of light transmittances of the lenses. For this reason, even a lens formed from synthetic silica glass manufactured by the above method still has insufficient light transmittance in use together with light having a specific wavelength of 250 nm or less.
When synthetic silica glass is used as a material of a photomask substrate, problems have been pointed out, including an increase in transmittance loss due to internal absorption or internal scattering of light, color center induced by the laser, degradation in optical performance due to heat or fluorescent light, and compaction that changes the density. Especially, the tendency becomes conspicuous when synthetic silica glass is used together with light having a wavelength of 190 nm or less.
Thus, an optical member such as a lens or photomask substrate formed from synthetic silica glass has insufficient optical characteristics such as UV resistance or light transmittance for light having a specific wavelength of 250 nm or less. When these optical members are used in a reduction projection exposure apparatus using a light source such as an ArF excimer laser (wavelength: 193 nm) or F2 laser (wavelength: 157.6 nm), line-width unevenness (print unevenness) in the pattern transfer process, and the like pose problems and make it very hard to achieve high resolution. For this reason, as a material of an optical member used together with light having a specific wavelength of 190 nm or less, single-crystal calcium fluoride has been examined because of its higher light transmittance and satisfactory UV resistance.
However, since single-crystal calcium fluoride has low resistance to a change in temperature, and is fragile and therefore readily breaks, damage easily occurs in the pattern formation process. In addition, since the linear expansion coefficient is as large as about 40 times that of silica glass, it is difficult to form a highly accurate mask pattern, and very strict temperature management is required for exposure processing.
The shorter the wavelength of light used in a reduction projection exposure apparatus becomes, the higher the optical performance required for an optical member such as a lens member or photomask member becomes. However, an optical member having desired optical performance, i.e., suitable for use in an apparatus using light having a wavelength of 250 nm or less and, more particularly, light having a wavelength of 190 nm or less has not been developed yet.
It is an object of the present invention to provide a UV synthetic silica glass optical member which has high light transmittance and UV resistance, achieves high resolution without generating any line-width unevenness (print unevenness) in the pattern transfer process, and therefore is suitable for use in a reduction projection exposure apparatus using light having a wavelength of 250 nm or less.
The present inventors have extensively studied to achieve the above object, consequently found that when in a silica glass optical member, the difference between the maximum value and the minimum value of transmittance [%/cm] per cm in thickness for the light in a predetermined direction within the plane perpendicular to the optical axis is 2.0%/cm or less, the above problems are solved, and completed the present invention.
More specifically, according to the present invention, there is provided a synthetic silica glass optical member used together with light having a specific wavelength of not more than 250 nm, wherein a difference between a maximum value and a minimum value of transmittance [%/cm] per cm in thickness for the light in a predetermined direction within a plane perpendicular to an optical axis is not more than 2.0%/cm.
According to the present invention, there is also provided a reduction projection exposure apparatus comprising an exposure light source for emitting light having a wavelength of not more than 250 nm, a photomask formed with an original image of pattern, an illumination optical system for irradiating the photomask with the light emitted from the light source, a projection optical system for projecting the pattern image output from the photomask onto a photosensitive substrate, and an alignment system for aligning the photomask and the photosensitive substrate,
wherein at least a part of lenses of the illumination optical system and projection optical system, and the photomask are formed from synthetic silica glass in which a difference between a maximum value and a minimum value of transmittance [%/cm] per cm in thickness for the light in a predetermined direction within a plane perpendicular to an optical axis is not more than 2.0%/cm.
As described above, in the optical member of the present invention, when the difference between the maximum value and the minimum value of transmittance [%/cm] per cm in thickness for the light in the predetermined direction within the plane perpendicular to the optical axis is set to 2.0%/cm or less, and the member is used as a lens of the optical systems of the reduction projection exposure apparatus, or photomask substrate, the decrease in light transmittance of the entire optical system is suppressed. In the photomask, high light transmittance can be obtained, and local thermal expansion of the member can be suppressed. Hence, high resolution can be attained without generating line-width unevenness (print unevenness) in the pattern transfer process.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.